Ars Magica has, hands down, the best magic system in an rpg for me. It also assumes that wizards are usually at the top of the heap, power-wise. Not invincible, mind you. It also has a couple ways to be fairly straight forward with magic if you don't want the complexity. I believe the 4th...
Ahh, keyboards...
Cult Outline
Myth / History; The entity's spiritual role, such as God of Farmers, Son of Earth Goddess
Nature / Place in Society; The temple's mundane role. Popular amongst farmers, Leaders of peasant revolts
Organization; Salient details of the temple itself and its...
How do you feel about sacrificing XP for magic item creation, with the caveat that others could make the sacrifice? The fighter can sacrifice the XP for the new sword, for example. That being an optional loss.
I found the scheme for cult write-ups used in RuneQuest to be quite useful. To be clear, my main game is D&D. I freely pillage ideas from other systems.
I would list the outline but I'm on my phone and my thumbs are not dexterus enough.
There were the Books of Vile Darkness and Exalted Deeds which looked in extremes of evil and good, respectively. For 3.5e, I believe there was something called liquid pain. While not a distillate of Evil, per se, it might be the closest you get. I don't recall anything similar in the Good book...
A world with a large city with strongly characterized neighborhoods, like Shadowrun's Seattle. Along with this some distant locales to explore like the Mystery Flesh Pit or the Long Stair. I wouldn't care about races/ancestries. Some notes on reinterpreting classes and a handful of new subclasses.
I considered this back in the AD&D days. The Rod of Seven Parts is one of my favorite artifacts, and the three sentences of lore about the Wind Dukes gave ma a lot of design space.